Modern computing devices include a variety of different types of integrated circuit (IC) chips including processors, memory devices, and controllers. On-chip and chip-to-chip interconnections within a computer are typically made with metal wires. As IC chips become more integrated, the wires become narrower and more closely spaced. This results in a higher resistance in the wires and a higher capacitance between the wires. The increase in resistance and capacitance may degrade electrical signals traveling through the wires. This signal degradation may reduce the performance of IC chips and the computing device as a whole.
To solve this problem, on-chip and chip-to-chip optical interconnections using light sources and waveguides have been proposed. In an optical interconnection system, an electrical signal from the IC chip is converted to an optical signal emitted by a light source. The light then travels over a waveguide to a detector, which converts the received light back to an electrical signal. The speed of the light in an optical interconnection is faster than the flow of electrons in a wire and scales linearly with the length of the optical interconnection. Also, optical interconnections can consume less electrical power than electrical interconnections as the interconnections become long.
Conventional optical interconnection systems generally require a light source that is not integrally formed with the IC chip. This is because Si and SiGe, the materials typically used to form IC chips, have not been considered suitable for forming integral light sources because they have an indirect band gap. Instead, external light sources made from direct band gap semiconductors are typically used in conventional optical interconnection systems. These light sources are separately packaged and aligned to the waveguide, as well as to other devices on the IC chip. This makes for a relatively expensive and complicated on-chip or chip-to-chip optical communication system.
Further complicating chip-to-chip communications is the limited number of contact pads that can be fabricated onto an IC chip. As IC chips increase in sophistication, more and more input/output leads are required to accommodate the larger number of bits and inputs/outputs for other applications.